1. Technical Field
The invention relates generally to integrated circuits, and more specifically, to circuits that incorporate fuse technology within integrated circuits.
2. Background Art
An important feature of integrated circuit designs is fuse technology. That is, a large number of integrated circuit designs now include fuses to implement certain features or to achieve desired results. For example, for many years dynamic random access memories (DRAMs) have included spare word lines and bit lines that can be substituted through fuse technology for faulty lines. On some integrated circuits, fuses are used to customize the chip function after manufacture. Fuses may also be used to trim precision components such as resistors or capacitors.
A fuse, has two possible states: the first state, when the fuse is intact; and the second or "blown" state, when the fuse is blown, or broken down. A fuse is typically blown through laser ablation or by forcing an excessive current through a narrow on-chip fuse wire. When either of these techniques are used, not only is the dielectric encapsulation, or protective covering around the fuse ruptured, which allows the metallic or similar material of the fuse to escape, but any other anti-corrosive material around the fuse is also ruptured.
Fuses are typically formed in the topmost layer of metal wires where they can be easily accessed for laser programming and from where the metallic material of the fuse can easily escape. In the past, the material used for fuses has been aluminum, which produces an oxide that is largely self passivating, thus preventing the fuse from "regrowing". Examples of this type of and similar types of fuses are found in the following U.S. patents: U.S. Pat. No. 5,291,139, "Circuit for Detection of the State of an Integrated Circuit Fuse in a Balanced Fuse Configuration", issued March 1994 to Fruhauf et al.; and U.S. Pat. No. 5,404,049, "Fuse Blow Circuit", issued April 1995 to Canada et al.
Future integrated circuits are progressing toward the use of other types of metals for wiring and fuses, such as copper, to allow for a smaller resistance in the circuit. Unfortunately, though, copper does not produce self-passivating oxides and thus is subject to dendritic growth, which can "regrow" a blown fuse; defeating the purpose of the fuse. Exposed copper is also subject to corrosion. Corrosion may enter the copper at the ruptured site and proceed to follow the wire to a junction where it will interrupt a device or circuit that is intended to remain conductive. Although the aforementioned patents disclose methods to detect the integrity of fuses, they do not disclose a method for preventing dendritic growth or corrosion in a blown fuse.